The present invention relates to a distance measuring apparatus which radiates a radio wave, receives reflected light from an object under measurement, and detects the object to detect the distance to the object and a relative speed.
As an example of the distance measuring apparatus, there is a distance measuring apparatus for an automotive vehicle. This distance measuring apparatus for an automotive vehicle radiates a radio wave to receive a reflected wave from a target such as a car, an obstacle or the like, and detects a propagation time of the radio wave, the magnitude of the reflected wave, a Doppler shift of the frequency, and the like to measure the distance to the target and a relative speed from the result.
Several methods are known for measurements of the distance to a target, and a relative speed. For example, a variety of methods have been described in xe2x80x9cTrend of Development of millimeter-wave radar for Vehicles,xe2x80x9d Journal of the Institute of Electronics, Information and Communication Engineers, pp977-981, October 1996.
Also, known radar methods include a two-frequency CW (Continuous Wave) method which switches two frequencies, an FMCW (Frequency Modulated Continuous Wave) method which performs a triangular modulation on a transmission frequency, a pulse method which transmits a pulse wave to measure the distance from a turnaround time of the pulse wave.
Further, JP-A-11-133143, JP-A-9-152477 and the like describe methods of separating and detecting a plurality of targets in an FMCW-based radar apparatus.
Further, researches have been under progress for a method of measuring a distance by switching a transmission frequency in a step-wise manner, receiving a reflected signal at each transmission frequency, and performing an inverse Fourier transform on these signals to derive time axis information, for example, as described in xe2x80x9cAbout Distance Measurement Processing for Stepped FM Code Radar,xe2x80x9d Transactions A of the Institute of Electronics, Information and Communication Engineers, Vol. 1, J81-A, No. 4, pp-490-495, April 1998.
Now, referring to FIG. 11, the principles of the two-frequency CW method will be described for measuring a relative speed of a target, making use of a Doppler shift, and switching two frequencies to measure the distance to the target from phase information of received signals at the respective frequencies.
With the two-frequency CW method using a single transmitter, two frequencies F1 (f1), F2 (f2) are alternately transmitted over time, as shown in FIG. 11(A). In the prior art, the frequencies F1, F2 are switched over time at a period of approximately 100 KHz, and a difference f between the two frequencies F1, F2 is approximately 300 KHz.
Next, the reception side fast Fourier transforms received signals at the respective transmission frequencies F1, F2 to find the frequency spectrum of a received beat signal. An example of the measured received frequency spectrum is shown in FIG. 11(B). When a target exists, a signal indicative of the target appears in a region of a frequency (Doppler frequency), which corresponds to a relative speed exhibited by the target, on the frequency spectrum, as shown in FIG. 11(B).
The two-frequency CW method has the ability to provide information on the target for each of the two transmission frequencies F1, F2. The radar method utilizing the Doppler frequency like the two-frequency CW method is capable of separating and detecting a plurality of targets which differ in relative speed from one another from the information on the frequency spectrum derived in the foregoing manner.
Then, the relative speed is calculated for each of the separated and detected targets from the Doppler frequency. The information on the frequency spectra for two received signals in the two-frequency CW method maybe shown invector representation based on the phase and amplitude, as can be seen in FIG. 11(C). The difference of phase angle between the two power spectra F1, F2 is proportional to the distance to the target.
When the power spectra F1, F2 are represented in complex signals Signal(1), Signal(2), the relationship between the distance (Range) and the phase difference of the two frequency is expressed by the following Equation 1 from the fact that the difference f between the transmission frequencies F1, F2 is known:
Range=(cxc2x7|xcex8|)(4xcfx80xc2x7xcex94f)xe2x80x83xe2x80x83(1)
where xcex94f=F2xe2x88x92F1;
xcex8=arg(Signal(1))xe2x88x92arg(Signal(2)); and
c is the velocity of light.
From the foregoing, the two-frequency CW method can calculate a relative speed with a target from the Doppler frequency, and can calculate the distance to the target from phase angle information.
However, distance measuring apparatuses utilizing the Doppler shift, as represented by the two-frequency CW method, separates and detects targets relying on the relative speed, so that such apparatuses experience difficulties in separating and detecting a plurality of targets having the same relative speed.
Therefore, for example, even if two obstacles actually exist, the apparatus may determine in some cases that only one obstacle exists.
It is an object of the present invention to provide a distance measuring apparatus which has a function of separating and detecting a plurality of targets, which are substantially equal in relative speed to one another, in a radar apparatus which utilizes the Doppler frequency.
To achieve the above object, the present invention is configured in the following manner.
(1) A distance measuring apparatus which radiates a radio wave, receives a reflected wave from an object under measurement, and detects the object under measurement, wherein the apparatus comprises transmitting means for continuously transmitting a first frequency signal for a predetermined time or more, continuously transmitting a second frequency signal having a predetermined frequency difference from the first frequency for a predetermined time or more, and transmitting a signal having a frequency difference of an integer multiple equal to or larger than twice the predetermined frequency difference from the first frequency over signals at N frequencies, where N is an integer equal to or larger than one, receiving means for measuring a Doppler frequency of the reflected wave from the object under measurement at each of the respective transmission frequencies of the first frequency signal, second frequency signal, and N frequency signals, and detection processing means for separating a plurality of objects under measurements to detect the respective objects.
(2) Preferably, in the foregoing (1), the N is one.
(3) Also, preferably, in the foregoing (1), (2), the detection processing means has a first separation/detection function for separating and detecting an object under measurement at each Doppler frequency of received signals, and a second separation/detection function for measuring phase information and amplitude information on received Doppler frequencies, and separating and detecting a plurality of object under measurements having substantially the same Doppler frequencies from the phase information and amplitude information.
(4) Also, preferably, in the foregoing (1), (2), (3), the transmitting means has a single transmitter, wherein a plurality of frequencies are periodically alternately transmitted by the single oscillator.
(5) A distance measuring apparatus which radiates a radio wave, receives a reflected wave from an object under measurement, and detects the object under measurement, wherein the apparatus comprises transmitting means for continuously transmitting a first frequency signal for a predetermined time or more, continuously transmitting a second frequency signal having a predetermined frequency difference from the first frequency for a predetermined time or more, and transmitting a signal having a frequency difference of an integer multiple equal to or larger than twice the predetermined frequency difference from the first frequency over signals at N frequencies, where N is an integer equal to or larger than one, the means having a first time region in which the first frequency signal and the second frequency signal are alternately transmitted over time, and a second time region in which the first frequency signal through (N+2)th frequency signal are respectively transmitted alternately over time, receiving means for measuring a Doppler frequency of the reflected wave from the object under measurement at each of the respective transmission frequencies of the first frequency, second frequency, and N frequencies, and detection processing means for separating a plurality of objects under measurement to detect the respective objects.